The present invention relates generally to seals and particularly to fabric seals for usage in low leakage static or dynamic sealing applications where pressure differentials and operating temperatures are moderate.
The increased worldwide demand for electricity leads to the necessity of designing large-scale, high-powered, high-efficiency, and low cost power generation turbomachines with high standards for reliability and performance. One of the primary concerns in a turbomachine design is its sealing performance because it has direct impact on machine operating reliability and efficiency. A number of seals are used in turbomachines such as gas/steam turbines, compressors, and generators for minimizing leakage flows. According to the movement of a seal relative to its sealing surface, seals can be classified as stationary seals affording leakage barriers around and between stationary components or rotating seals affording leakage barriers between stationary and rotating components. Over the years, various techniques have been developed to improve both stationary and rotating seals. Much of this development has occurred in the turbine industry where parasitic flow control is critical.
As a circumferential contacting seal, brush seals have been widely used in turbomachines due to their distinguishing characteristics, i.e., lower leakage flow rate compared with labyrinth seals and seal flexibility for accommodating excursions at the interface without excessive wear and loss of sealing capability. However, brush seals formed of brush bristles are usually expensive and have exhibited hysterisis and were difficult to manufacture. The crucial seal parameters such as cant angle and bristle height depend on the process capability or manufacturing technique and are very difficult to control. A conventional brush seal is made by either folding bristles over a metal plate or sandwiching bristles between a pair of ring-shaped metal support plates and welding ends of the bristles and plates to one another adjacent common edges thereof. Since the metal bristle holder is usually machined with a tight tolerance, a brush seal can fit only a specific sealing dimension. This can result in very high costs for brush seals in tooling, manufacturing, and installation and leads to a long cycle time in brush seal fabrication.
According to a preferred embodiment of the present invention, there is provided a flexible seal formed of a non-woven fabric disposed between a pair of strips, preferably also formed of a fabric material. Preferably, a continuous non-woven fabric layer is folded onto itself and is secured, for example, by adhesive along the fold lines to itself or to the strips to form a plurality of cells extending longitudinally between opposite sides of the seal. Consequently, the cells lie between the pair of fabric strips and the seal responds substantially freely to forces acting on the seal. That is, generally rectilinear cells which are collapsible and expandable in a direction between the two sealing surfaces are provided, the cells having dimensions varying from hundreds of micrometers to several centimeters.
In a particular application of the present invention, one of the strips may, for example, be adhered by adhesive to a stationary component, i.e., a stator surface. The other strip is located adjacent the rotating surface. Because the non-woven fabric forming the cells lies between the strips, the seal responds substantially freely to applied forces. Thus, any radial excursion of the rotating surface is accommodated by the collapsing and expansion of the cells of the seal. The strip in contact with the rotating surface may have a wear-resistance material applied to its surface. For example, a coating of Teflon or a near frictionless carbon coating can be applied to that strip.
As will be appreciated, the seals can be formed of one or more layers of cells and hence the thickness of the seal can be varied depending upon the sealing application. For example, for certain applications, the fabric layers may be formed of a mixture of metallic and non-metallic fibers enabling the seal for use in intermediate to high pressure applications. Various other design parameters may be altered depending upon the application such as fabric construction, material, fabric layer thickness, cell construction and dimensions. One particular advantage of the present seal is that in contrast to brush seals having bristles which are laid at certain angles to the shaft surface to facilitate seal flexibility and avoid excessive fiber stress during shaft excursion, the present seal is independent of the direction of rotation of the shaft. The risk of seal damage due to reverse rotation is thus eliminated. Secondly, seal flexibility is adjustable. For example, seal flexibility may be increased by increasing the cell size and/or the number of cell layers, as compared with a less flexible seal with decreased cell size and/or cell layers. To create stiffness in the seal, the seal can be constructed with a half cell layer or a one cell layer. For applications which require high seal stiffness, inserts may be placed in the seal cells. Such inserts may take numerous forms, for example, thin walled pipes formed of rubber or plastic, such as aerated plastic sticks or helical wire coils.
A third immediate advantage of the present seal is that it provides solutions for applications having large sealing dimensions and/or irregular sealing geometry. Conventional seals require machining brush holders precisely with tight tolerances. As a result, each seal can serve only a specific sealing dimension. As a sealing dimension becomes large, it becomes very difficult to make the seal. The present seal can readily and easily fit different sealing dimensions and complex sealing geometry.
In a preferred embodiment according to the present invention, there is provided a seal between first and second spaced components comprising generally parallel fabric layers spaced from one another with a first layer thereof for securement to the first component and a second layer thereof disposed adjacent the second component and a non-woven fabric secured to and between the first and second layers, the non-woven fabric defining a plurality of cells between the first and second layers extending in a direction generally parallel to the layers and perpendicular to opposite sides of the seal, the non-woven fabric biasing the second layer in a direction towards the second component.
In a further preferred embodiment according to the present invention, there is provided in an apparatus including a stationary component and a rotatable component about an axis, a seal between the components including a first layer of fabric secured to the stationary component and a second layer of fabric spaced from the first fabric layer and about the rotatable component, a non-woven fabric between and secured to the layers, the non-woven fabric defining a plurality of multi-sided generally annular cells extending between the layers about the axis and in planes generally normal to the axis.